The present invention relates to a process for sweetening of LPG, light petroleum distillates by liquid-liquid extraction using metal phthalocyanine sulphonamide catalyst.
Particularly, the invention relates to a process for sweetening of LPG, light petroleum distillates like pentanes, light straight run naphtha (LSRN), comprising of liquid-liquid extraction of the mercaptans contained therein by alkali solution and regeneration of the mercaptan containing alkali solution by oxygen using metal phthalocyanine sulphonamide catalyst, whereby the mercaptans are converted to corresponding disulphides and the regenerated alkali solution can be reused for mercaptan extraction.
Metal phthalocyanine sulphonamide catalyst has been prepared by a procedure as discussed and described in our co-pending Indian Patent Application No 1032/DEL/2000 corresponding to U.S. patent application Ser. No. 09/804,994.
It is known that the presence of mercaptans in the petroleum products like LPG, naphtha, gasoline, kerosene, ATF etc is highly undesirable due to their foul odour and highly corrosive nature. These are also poisonous to the catalysts and adversely affect the performance of tetraethyl lead as octane booster. Although there are several processes known for the removal of mercaptans from petroleum products, the most common practice is to oxidize the mercaptans present, to less deleterious disulphides with air in the presence of a catalyst. Generally, the lower mercaptans present in LPG, pentanes, LSRN and light thermally cracked naphtha are first extracted in alkali solution and then oxidized to disulphides with air in the presence of a catalyst. The disulphides, being insoluble in alkali solution is separated out from the top and the alkali is regenerated. In the liquid-liquid sweetening the lower mercaptans present in petroleum products like pentanes. LSRN, FCC cracked naphtha etc. are converted to disulphides by direct oxidation with air in the presence of alkali solution and catalyst. The higher molecular weight mercaptans present in petroleum products like heavy naphtha, FCC gasoline, ATF and kerosene are oxidized to disulphides with air in a fixed bed reactor containing catalyst impregnated on a suitable support like activated carbon (Catal. Rev. Sci. Eng. 35(4), 571-609, 1993).
It is also well known that the phthalocyanines of the metals like cobalt, iron, manganese, molybdenum and vanadium catalyze the oxidation of mercaptans to disulphides in alkaline medium. Among these cobalt and vanadium phthalocyanines are preferred. As the metal phthalocyanines are not soluble in aqueous medium, for improved catalytic activity their derivatives like sulphonated and carboxylated metal phthalocyanines are used as catalyst for sweetening of petroleum fractions. For example use of cobalt phthalocyanine monosulphonate as the catalyst in the fixed bed sweetening of various petroleum products (U.S. Pat. Nos. 3,371,031; 4,009,120; 4,207,173; 4,028,269; 4,087,378; 4,141,819; 4,121,998; 4,124,494; 4,124,531) and cobalt phthalocyanine disulphoante (U.S. Pat. No. 4,250,022) tetra sulphonate (U.S. Pat. No. 2,622,763) and the mixture thereof (U.S. Pat. No. 4,248,694) as catalysts for liquid-liquid sweetening and alkali regeneration in mercaptan extraction of light petroleum distillates has been reported. The use of phenoxy substituted cobalt phthalocyanine as sweetening catalyst (Ger Offen 3,816,952), cobalt and vanadium chelates of 2, 9, 16, 23-tetrakis (3,4-dicarboxybenzoyl) phthalocyanine as effective catalyst for both homogeneous and fixed bed mercaptan oxidation (Ger Offen 2,757,476; Fr. Demande 2,375,201) and cobalt, vanadium chelates of tetrapyridinoporphyrazine as active catalysts for sweetening of sour petroleum distillates (Ger Offen 2,441,648) has also been reported.
It is well known that the catalysts used for the sweetening of LPG and light petroleum distillates like pentanes, LSRN etc. by liquid-liquid mercaptan extraction and alkali regeneration are di-, tri- and tetra sulphonates of metal phthalocyanines particularly those of cobalt and vanadium phthalocyanines; cobalt phthalocyanine sulphonates being specially preferred. The cobalt phthalocyanine sulphonates, differ in activity and in their solubility characteristics depending upon the number of sulphonate functionalities leading to problems in their use as catalysts.
Cobalt phthalocyanine disulphonate a commonly used catalyst in sweetening of LPG and light petroleum fractions by liquid-liquid mercaptan extraction and alkali regeneration is extremely dusty in the dry fine powder form and causes handling problem. To overcome this problem cobalt phthalocyanine disulphonate is admixed with water and commonly used as a slurry. However, with insufficient mixing the cobalt phthalocyanine disulphonate precipitates out from the slurry. Moreover, even if the slurry is mixed sufficiently, it retains the cobalt phthalocyanine disulphonate in suspension for a particular length of time only, beyond which the slurry becomes extremely viscous and may form gel, making it very difficult to remove the material from packaging. Cobalt phthalocyanine tetrasulphonate, on the other hand, is highly soluble in water and its use can eliminate precipitation and gel forming problems associated with the use of cobalt phthalocyanine disulphonate. However, it is reported that cobalt phthalocyanine tetrasulphonate has lower catalytic activity than cobalt phthalocyanine disulphonate (U.S. Pat. No. 4,885,268).
In one of our applications 1032/Del/2000 corresponding to U.S. patent application Ser. No. 09/804,994, we reported an improved process for the preparation of metal phthalocyanine sulphonamide catalyst useful for sweetening and obviates the drawback as detailed above.
The main objective of the present invention is to provide a process for sweetening of LPG, light petroleum distillates by liquid-liquid extraction and alkali regeneration using metal phthalocyanine sulphonamide catalyst, which obviates the drawbacks as detailed above.
Accordingly the present invention provides a process for sweetening of LPG, light petroleum distillates by liquid-liquid extraction using metal phthalocyanine sulphonamide catalyst which comprises extracting the mercaptanes contained in LPG, light petroleum distillate like pentanes, light straight run naphtha by liquid-liquid extraction using an aqueous or alcoholic solution of alkali metal hydroxide of concentration ranging between 1 wt % to 50 wt % in the presence of a metal phthalocyanine sulphonamide catalyst in the concentration ranging from 5-4000 ppmw, at a temperature ranging from 10xc2x0 C. to 80xc2x0 C., at a pressure ranging from 1 kg/cm2-50 kg/cm2 in a continuous or batch manner, converting the mercaptanes present in above said extract into corresponding disulphides by passing air, oxygen or any oxygen containing gas at the above same temperature and pressure, regenerating the alkali solution containing catalyst for recycling by separating the upper layer of disulphides from said alkali solution of catalyst.
In an embodiment of the present invention metal phthalocyanine sulphonamide catalyst used in selected from the group consisting of cobalt, manganese, nickel, iron, vanadium phthalocyanine sulphonamide and their N-substituted sulphonamide derivatives most preferably cobalt phthalocyanine sulphonamide.
In an embodiment of the present invention the alkali solution used for mercaptan extraction is selected from aqueous or alcoholic solution of alkali metal hydroxides selected from the group consisting of sodium hydroxide, potassium hydroxide, lithium hydroxide, rubidium hydroxide, and cesium hydroxide most preferably aqueous solution of sodium and potassium hydroxide.
In yet another embodiment of the present invention the concentration of the alkali solution used is preferably in the range 7% to 25% by weight.
In yet another embodiment of the present invention the metal phthalocyanine sulphonamide catalyst used is preferably in the concentration ranging between 10 to 1000 ppmw related to alkaline reagent.
In yet another embodiment of the present invention the conversion of mercaptanes to disulphides is effected preferably at 35xc2x0 C. to 60xc2x0 C.
In yet another embodiment of the present invention the conversion of mercaptanes to disulphides is effected preferably at 1 kg/cm2 to 15 kg/cm2 pressure.
In yet another embodiment of the present invention the conversion of mercaptanes to disulphides is preferably effected by air.
In still another embodiment of the present invention the regeneration of alkali solution is effected with the mercaptide sulphur ranking from 10 ppmw to 40,000 ppmw in feed stocks.
In the sweetening process herein contemplated the undesirable mercaptans contained in LPG and light petroleum distillates like, pentanes, LSRN are extracted with alkali solution containing metal phthalocyanine sulphonamide catalyst through a counter current liquid-liquid extraction. The sweetened petroleum distillate is then passed through an alkali settler and sand filter to remove entrained alkali. The mercaptans and catalyst containing alkali solution obtained from the extractor is oxidized by oxygen or oxygen containing gas like air in on oxidizer whereby the mercaptans present in alkali solution are converted into corresponding disulphides and alkali is regenerated. The disulphide oil being insoluble separates from alkali solution as upper layer and is drained. The regenerated alkali solution is reused for mercaptan extraction.
In the sweetening process with this catalyst system extraction of mercaptans from light petroleum distillates can be effected at 10xc2x0 C. to 80xc2x0 C. but the preferred range is 10xc2x0 C. to 40xc2x0 C. The extraction can be effected at a pressure from ambient to 50 kg/cm2 or more with the preferable pressure range ambient to 20 kg/cm2. The alkali solution used in the extraction is aqueous/alcoholic solution of alkali metal hydroxide such as sodium hydroxide, potassium hydroxide, lithium hydroxide, rubidium hydroxide, cesium hydroxide, aqueous solution of sodium hydroxide and potassium hydroxide being preferred. The concentration of the alkali solution used is 1% to 50% the preferred range being 7 to 25%.
The sweetening process is effected with the metal phthalocyanine sulphonamide catalyst like cobalt, manganese, nickel, iron and vanadium phthalocyanine sulphonamide and their N-substituted derivatives, the preferred catalyst is cobalt phthalocyanine sulphonamide. The catalyst is used in the concentration 1 to 1000 ppmw related to alkali solution, the preferred range is 10-1000 ppmw.
The regeneration of mercaptans containing alkali solution with metal phthalocyanine sulphonamide catalyst is effected at ambient to 90xc2x0 C. temperature. The preferred range being 35xc2x0 C. to 60xc2x0 C.
The regeneration of alkali solution is effected at atmosphere to 50 Kg/cm2 pressure, the preferred range being 1-15 Kg/cm2.
The regeneration of alkali solution is effected by air, oxygen or any other oxygen containing gas, air being especially preferred.